ype of open web steel jois

huangyhg

type of open web steel joist?
i am having difficulty identifying a joist which is at least 35 years old.  top chord consist of two channels (estimated at 1.5" deep by 1/4" thick). bottom chord consists of two angles (leg size estimated at 1.5" by 1/4" thick).  i have no engineering data and info on the type of joist.  current material on internet does not provide much help.  
was there a joist tag on the joist?
jae, i dont know.  where underneath the roof would you suggest to look for the joist tag?   and would each joist be tagged?  by the way the two channels are 2.5" deep and not 1.5" as originally stated.
look for a small piece (say 3/4" x 6" roughly) of stamped sheet metal tied with wire near one end of the joists.  the wire is usually wrapped around bott chord or bottom of a web   
samdamon, yes to the "added load" question.  all calculations are done except that the type of joist was assumed to be a 24k4 serie.  the depth of the roof joists is 24", web rod 3/4" dia  and width of the bottom chord is 3-5/8", consequently my assumption puts me in a ball park but i am still off the mark because current literature describes k series having angles for top and bottom chords.  i could be dealing with kcs serie but so far i have not been able to verify this either.
with my current assumption, the added loads are not significant to the roof support system, however i can not submit what i have to the buiding inspector until i can identify the type of joists.
35 years old would put you about 1970, you are more likely into a j or h series, j series have a max allowable tensile of 22 ksi, h series have 30 ksi, k series did not exist.  all j,h, and k series have a 2 1/2" bearing depth,the long span joists have a 5" bearing depth.  verify the web diameters in a few spots as this can vary at the ends. if your load does not work assuming j series, you may have to have a sample tested for tensile strength. there really is no other way to tell if you can't find the tag.
try to get a copy of the steel joist digest, they have come out with the 75 year digest 1928-2003 and covers all joists manufactured with specifications and load tables.
chicopee, i believe kcs type joists are a relatively recent innovation and were not available until well after your orig construction date.  shapes used as top and bott chords are not any help in narrowing down your joist size because unfortunately different joist fabricators were allowed to use different shapes to achieve the required propeties.  there was no standardization in the shapes used. your best bet is still to find a tag if you can.
chicopee, are you adding point loads? what percentage is the bending moment being increased?
tags r ok and for the most part u can find them .... related more to erection/placement and fabrictor, rather than load capacity .... but at the end of the day ... and this seems to be a never ending problem  here .... how do you determine the structural capacity?  sji aside, how do resolve shear requirements when you analizing a pin jointed truss? ... panel points being < 24 inches.
thanks to the responders for their inputs.  i finally got to find out the type of joist by snaking my way thru the suspended ceiling and all the electrical wiring. the tag on one joist indicated the following" ganon steelworks;made in canada;j3- cumber". so obviously we have a 24j3 joist but i coul not figure out the word "cumber", so if any one knows what it means let me know.  also i would appreciate a copy of a load table instead of using my stress calculations.
chicopee,
the older aisc steel manuals contain joist information including load tables and fy's.  check the light blue manual - i think thats the seventh addition.